It is known to combine a vapor compression system with a Rankine cycle system. See, for example, U.S. Pat. No. 6,962,056, assigned to the assignee of the present invention. In the U.S. Pat. No. 5,761,921 power generated in the Rankine cycle is applied to drive the compressor of the vapor compression cycle, and the combined systems operate on three pressure levels, i.e. the boiler, condenser and evaporator pressure levels.
In the U.S. Pat. No. 5,761,921, the condenser (heating) capacity includes two main components, the boiler heat exchanger capacity and the evaporator capacity, thereby increasing the heating effect for a given thermal input. However, such heat pumps are very sensitive to ambient temperature in a heating mode of operation, since the heating capacity decreases as ambient temperature decreases. This occurs because the density of vapor refrigerant at the compressor suction and the compressor volumetric efficiency are reduced as the ambient temperature decreases. Also, system operation in cooling is somewhat inefficient, and the efficiency drops when the ambient temperature increases.
Further, the combined systems cannot use trans-critical refrigerants, since there is no condensed liquid available downstream of the gas cooler for pumping through the Rankine circuit.